1. Field of the Invention
This invention relates to a diamond sintered body or compact for tools and a process for the production of the same, and more particularly, it is concerned with a diamond sintered body or compact for tools, comprising coarse diamond grains and a binder containing ultra-fine diamond grains and a process for the production of the same.
2. Description of the Prior Art
At the present day, diamond compacts containing more than 70% by volume of diamond have commercially been sold for wire-drawing dies or cutting tools of nonferrous metals, plastics or ceramics. Above all, fine diamond grain compacts are favourably commented because when using as a die for drawing a relatively soft wire rod such as copper wire, there is obtained a wire with a very smooth surface after drawn thereby. However, no one has developed a diamond compact with satisfactory properties for drawing a high hardness wire rod such as brass-plated high carbon steel wire. In the case of using these commercially sold diamond compacts as cutting tools of ceramics or drill bits, a compact consisting of fine diamond grains has a problem as to wear resistance and a compact consisting of coarse diamond grains tends to be broken to such an extent that the reuse be impossible. Other tool materials comprising the above described diamond compact part bonded to a substrate of cemented carbide have also been marketed and favourably commented, in particular, as a cutting tool of Al-Si alloys or high hardness Cu alloys in spite of their higher prices.
The inventors have made studies on the properties of these tool materials and have found that these materials comprise as a predominant component diamond crystals contacted with each other to form a skeleton structure having a more excellent wear resistance than the commonly used cemented carbides. However, it is also found that these excellent diamond compacts, on the other hand, have various disadvantages. The first disadvantage is that the price is too high though the wear resistance is very excellent, and moreover, regrinding involves also great expense. Because of the high price, even the insert worn once cannot be thrown away without regrinding, while inserts of cemented carbides are ordinarily of throw-away type. When an insert of diamond compact is subjected to a regrinding test based on this thought, this seems rather to dress a diamond wheel used therefor than to grind the insert. That is, the regrinding efficiency is very low and the consumption of the diamond wheel is very large.
The second disadvantage is that when a nonferrous alloy is subjected cutting and the worked surface is observed, for example, the surface roughness is not so fine as using a cutting tool of natural diamond single crystal and no one finds a beautiful finished surface called mirror surface. In addition, when small articles such as watch parts or thin workpieces are subjected to cutting, problems arise that the cutting resistance is too large to hold the dimensional precision and the workpieces tend to be deformed. In the commercially sold diamond compacts, as set forth above, diamond crystals are contacted with each other to form a skeleton structure and there is cobalt among diamond grains with a grain size of 3-8 .mu.m. When the edge of a cutting tool using this compact is observed, there is found roughness substantially corresponding to the size of the crystal grains. This is considered to be one reason why a beautiful finished work surface is hardly obtained. Furthermore, the metallic Co binder phase present among diamond grains tends to adhere to a workpiece metal, which is a problem, in particular, in a case where a finished work surface like mirror surface is required.
In order to solve these problems, one of the inventors has developed and proposed a compact with a decreased content of diamond grain and a compact consisting of diamond grains with a size of less than 1 micron (U.S. Pat. Nos. 4,171,973 and 4,303,442). These compacts are surely improved in workability to be ground as well as in edge sharpness, but it is found that they show some drawbacks as to wear resistance and adhesivity resistance depending upon the variety of workpieces.